Digital image scanner with a variable aperture lens and multiple scanning speeds

ABSTRACT

A scanner is provided with a variable aperture lens system. High resolution scans use a relatively small aperture size, and scanning speed is relatively slow. Low resolution scans use a relatively large aperture size, and scanning speed is increased. Fast scans are limited to lower sampling rates, which in turn permit more optical blurring relative to high sampling rates. Accordingly, the incremental cost of the larger aperture is minimized by permitting the lens aberrations specifications to be relaxed at larger apertures. Preferably, an electronic variable aperture is provided, for example, by use of electronically controlled polarization plates or by use of electrochromic substances.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application is a continuation of U.S. applicationSer. No. 09/614,579 (filed Jul. 12, 2000), which is acontinuation-in-part of Ser. No. 09/257,459 (filed Feb. 25, 1999,abandoned). application Ser. Nos. 09/614,579 and 09/257,459 are herebyincorporated herein by reference.

FIELD OF INVENTION

[0002] This invention relates generally to digital image scanners,copiers, and facsimile machines and more specifically to system in whichspeed and sampling rate are interrelated through use of a variableaperture lens.

BACKGROUND OF THE INVENTION

[0003] Image scanners convert a visible image on a document orphotograph, or an image in a transparent medium, into an electronic formsuitable for copying, storing or processing by a computer. An imagescanner may be a separate device or an image scanner may be a part of acopier, part of a facsimile machine, or part of a multipurpose device.Reflective image scanners typically have a controlled source of light,and light is reflected off the surface of a document, through an opticssystem, and onto an array of photosensitive devices. The photosensitivedevices convert received light intensity into an electronic signal.Transparency image scanners pass light through a transparent image, forexample a photographic positive slide, through an optics system, andthen onto an array of photosensitive devices.

[0004] In general, image scanners use an optical lens system to focus animage onto an array of photosensors. Photosensor arrays typically havethousands of individual photosensitive elements. Each photosensitiveelement, in conjunction with the scanner optics system, measures lightintensity from an effective area on the document defining a pictureelement (pixel) on the image being scanned. Optical sampling rate isoften expressed as pixels per inch (or mm) as measured on the imagebeing scanned. For opaque images, for example, photographs or printedpages, a typical scanner optical sampling rate is 600 pixels per inch(24 pixels per mm).

[0005] One important specification for scanning is the overall timerequired to scan an image. One important factor is the data transferrate between the scanner and a host computer. High sampling ratesgenerate large quantities of data, and at high sampling rates the datatransfer rate may limit overall scanning time. Another important factorthat can affect the overall time required to scan an image is the sensorexposure time. Typically, in a photosensor circuit, current generated bylight is used to change the charge on a capacitor. A finite time isrequired to change the charge on a capacitor sufficiently to ensure anacceptable signal to noise ratio. At lower sampling rates, sensorexposure time may limit the overall time required to scan an image. Therequired sensor exposure time may be reduced by increasing the intensityof illumination on the document, or by increasing the aperture size ofthe optics system (thereby capturing more of the light reflected fromthe document). Either approach adds cost. In particular, the cost of alens system increases substantially with aperture size, partly becausecontrolling lens aberrations becomes more difficult and expensive as thelens aperture size increases. There is a need for reducing exposure timein a digital image scanner without substantially increasing cost.

SUMMARY OF THE INVENTION

[0006] In a scanner in accordance with the invention, a variableaperture lens system is provided. For high sampling rate scans, whereoverall scan time may be limited by data transfer rate, a relativelysmall aperture size is used. A small aperture size minimizes lensaberrations. However, a small aperture size reduces the light capturedfor the sensor array, and as a result, the exposure times are relativelylong. For low sampling rates, where sensor exposure time may limitscanning time, a relatively large aperture size is used. With a largeraperture size, sensor exposure time is reduced, and scanning speed maybe increased until some other factor, such as data transfer rate, limitsoverall scanning time. Fast scans are limited to lower sampling rates,which in turn permit more optical blurring relative to high samplingrates. Accordingly, the lens aberrations specifications are relaxed atlarger apertures to minimize the incremental cost of the largeraperture. The invention enables higher scanning speeds in a mode inwhich the scanner can take advantage of higher scanning speeds. That is,in a mode in which data transfer rate is not the limiting factor (lowresolution), the invention enables faster scanning. The variableaperture may be mechanical, electromechanical, or electronic (forexample, by using an electronically controlled polarization plate, orelectrochromic substances).

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is a block diagram side view of an image scanner inaccordance with the invention.

[0008]FIG. 2 is flow chart of a method in accordance with the invention.

[0009]FIG. 3 is flow chart of an alternative method in accordance withthe invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

[0010]FIG. 1 illustrates a simplified image scanner. A lamp 100illuminates a document 102. Light reflected from the document 102 passesthrough a lens system 104 and is focused onto a sensor array 108. Thelens system 104 includes a variable aperture element (for example, aniris diaphragm) 106. The image scanner has a processor based controller(not illustrated) and the aperture size of the iris 106 is controllableby the controller. Typically, image scanning lens systems have multipleelements. Lens systems for image scanning typically do not include avariable aperture element. Image scanners typically have other opticalsystem design considerations that are not relevant to the present patentdocument. For example, image scanners may have folded optical paths, andcolor image scanners have color separation devices (for example, filtersor beam splitters).

[0011] Sampling rate is the number of samples, in pixels per inch (orpixels per mm), that are created by a scanner, per linear distance.Optical sampling rate is the sampling rate defined by the opticalmagnification and the pitch of the sensor array. Sampling rates otherthan the optical sampling rate are obtained by interpolation of pixeldata, or by decimation of pixel data. Resolution is the degree to whichthe scanner distinguishes detail in an object; it is affected bysampling rate, but also by other aspects of a scanner such as opticsquality, mechanical stability, temperature, humidity, electronicbandwidth, and image processing. Modulation transfer function is ameasure of a scanner's optical frequency response with scanning linepairs having a spatial frequency that is within the limits of thescanner. For further discussion of scanning optical parameters, seeRobert G. Gann, Desktop Scanners: Image Quality Evaluation, PrenticeHall, 1998.

[0012] In an image scanning system in accordance with the invention, arelatively small lens aperture is used when relatively high samplingrates are requested, or when relatively slow scanning speed is selected.A relatively large lens aperture is used when relatively low samplingrates are requested, or when a relatively fast scanning speed isrequested. Given a specified modulation transfer function of an imagescanning system, the lens system must be designed so that thecontribution of the optical aberrations are sufficiently small to enablethe specified modulation transfer function. In the present invention,however, large apertures are used only for low sampling rates, and lowsampling rates permit a reduced spatial bandwidth for the modulationtransfer function. Therefore, the specifications for aberrations for thelens system are relaxed for large apertures. As a result, faster scanscan be performed (the larger aperture captures more light, therebydecreasing the time required to change the charge on photosensorcapacitances), without requiring a large increase in the cost of thelens system.

[0013] The variable aperture element 106 may be mechanical. That is, ascanner user could manually select an aperture just as aperture ismanually selected on many camera lenses. Alternatively, the variableaperture element could be electromechanical, with the controllerselecting an aperture. Again, the camera industry provides numerousexamples of electromechanical apertures.

[0014] Preferably, for highest reliability, the variable apertureelement 106 is electronic, with no moving parts. One example embodimentof an electronic aperture is to use polarizing optics. For example, twopolarization layers or plates may be used in conjunction with anelectronically controllable phase retarder. A first polarization plateor layer, transmitting light primarily of a singular polarization, maybe placed anywhere in the optical path. The phase retarder, placedbetween the two polarization plates or layers, may be electronicallycontrolled to either transmit light with no change, or to retard thephase of transmitted light by ninety degrees. A second polarizationplate or layer may have an annular ring that transmits light primarilyof the same singular polarization as the first polarization plate orlayer. The inner diameter of the annular ring defines the smalleraperture, and the outer diameter of the annular ring defines the largeraperture. When the phase retarder is controlled to transmit light withno change, the two polarization plates transmit light of onepolarization. However, if phase retarder retards the phase oftransmitted polarized light by ninety degrees, then most of the lightcannot pass through both the first polarization plate and the polarizedannular ring of the second polarization plate. One commerciallyavailable example of an electronically controlled retarder is a liquidcrystal variable retarder available from Meadowlark Optics, P.O. Box1000, 5964 Iris Parkway, Frederick, Colo., 80530. Another example is aferroelectric liquid crystal cell, available from Displaytech, Inc.,2602 Clover Basin Drive, Longmont, Colo., 80503.

[0015] An alternative embodiment for a electronic aperture is to useelectrochromic substances. Electrochromic substances change theirtransmittance to light in response to a variation of electrical current,voltage, or magnetic field. These substances may be implemented as aseparate iris element as illustrated in FIG. 1 (some electrochromicdevices are implemented as a liquid or gel between transparentelectrodes), or may be implemented as a film used to directly coat theouter area of a lens surface. There are many electrochromic substancesdescribed in the optics literature. Two specific examples ofelectrochromic substances being used for electronic apertures for lenssystems may be found in U.S. Pat. Nos. 4,526,454 and 5,471,339. In eachof the cited patents, an electronic diaphragm is described that iscapable of having more than two selectable apertures for lighttransmittance. While a multiple aperture device could provide manychoices for speed in an image scanner, it may be suitable in an imagescanner to simply provide a choice of two apertures for lighttransmittance. That is, the aperture, whether polarizing orelectrochromic, may have a transparent inner area surrounded by anelectronically controlled annular or ring shaped area.

[0016]FIG. 2 is a flow chart illustrating a method in accordance withthe invention. At step 200, an operator selects a sampling rate. If alow sampling rate is selected (test 202), the iris is set to a largeaperture size and the scanning speed is set to a high speed (step 206).If a high sampling rate is selected, the iris is set to a small aperturesize, and the scanning speed is set to a low speed. The document orother image is then scanned using the selected sampling rate, aperturesize, and speed (step 208).

[0017]FIG. 3 is a flow chart illustrating an alternative method inaccordance with the invention. At step 300, an operator selects ascanning speed. If a high speed is selected (test 302), the iris is setto a large aperture size and the sampling rate is limited to arelatively low rate (step 306). If a low speed is selected, the iris isset to a small aperture and the sampling rate is not limited (step 304).The document or other image is then scanned using the selected samplingrate, aperture size, and speed (step 308).

[0018] The foregoing description of the present invention has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and other modifications and variations may be possible inlight of the above teachings. The embodiment was chosen and described inorder to best explain the principles of the invention and its practicalapplication to thereby enable others skilled in the art to best utilizethe invention in various embodiments and various modifications as aresuited to the particular use contemplated. It is intended that theappended claims be construed to include other alternative embodiments ofthe invention except insofar as limited by the prior art.

What is claimed is:
 1. An optical image scanner comprising: a lenssystem; an iris within the lens system, the iris having at least a firstaperture size and a second aperture size, the second aperture size beinglarger than the first aperture size; and the iris set to the firstaperture size when scanning at a first speed and the iris set to thesecond aperture size when scanning at a second speed, the second speedfaster than the first speed.
 2. The optical image scanner of claim 1,further comprising: the lens system having sufficiently low aberrationsat the first aperture size to enable a specified modulation transferfunction; and the lens system having aberrations at the second aperturesize such that a resulting modulation transfer function is worse thanthe specified modulation transfer function.
 3. A method for scanningcomprising: selecting a first sampling rate or a second sampling rate,the second sampling rate being higher than the first sampling rate;scanning using a first aperture size when the sampling rate is the firstsampling rate; scanning using a second aperture size when the samplingrate is the second sampling rate, the first aperture size being largerthan the second aperture size.
 4. A method for scanning comprising:selecting a first scanning speed or a second scanning speed, the secondscanning speed being faster than the first scanning speed; scanningusing a first aperture size when the scanning speed is the firstscanning speed; scanning using a second aperture size when the scanningspeed is the second scanning speed, the first aperture size beingsmaller than the second aperture size.